The disclosure relates to electrochemical cells, and more particularly, to an electrode catalyst composition, electrode, and membrane electrode assembly for use in electrochemical cells.
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells. A proton exchange membrane electrolysis cell can function as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gas, and can function as a fuel cell by electrochemically reacting hydrogen with oxygen to generate electricity. Referring to FIG. 1, which is a partial section of a typical anode feed electrolysis cell 100 (xe2x80x9ccell 100xe2x80x9d), process water 102 is fed into cell 100 on the side of an oxygen electrode (anode) 116 to form oxygen gas 104, electrons, and hydrogen ions (protons) 106. The reaction is facilitated by the positive terminal of a power source 120 electrically connected to anode 116 and the negative terminal of power source 120 connected to a hydrogen electrode (cathode) 114. The oxygen gas 104 and a first portion 108 of the process water exit cell 100, while protons 106 and a second portion 110 of process water migrate across a proton exchange membrane 118 to cathode 114 where hydrogen gas 112 is formed.
Another typical water electrolysis cell using the same configuration as is shown in FIG. 1 is a cathode feed cell, wherein process water is fed on the side of the hydrogen electrode. A portion of the water migrates from the cathode across the membrane to the anode where hydrogen ions and oxygen gas are formed due to the reaction facilitated by connection with a power source across the anode and cathode. A portion of the process water exits the cell at the cathode side without passing through the membrane.
A typical fuel cell uses the same general configuration as is shown in FIG. 1. Hydrogen gas (from a pure hydrogen source, hydrocarbon, methanol, or other hydrogen source) is introduced to the hydrogen electrode (the anode in fuel cells), while oxygen, or an oxygen-containing gas such as air, is introduced to the oxygen electrode (the cathode in fuel cells). Water can also be introduced with the feed gas. Hydrogen gas electrochemically reacts at the anode to produce protons and electrons, wherein the electrons flow from the anode through an electrically connected external load, and the protons migrate through the membrane to the cathode. At the cathode, the protons and electrons react with oxygen to form water, which additionally includes any feed water that is dragged through the membrane to the cathode. The electrical potential across the anode and the cathode can be exploited to power an external load.
In other embodiments, one or more electrochemical cells can be used within a system to both electrolyze water to produce hydrogen and oxygen, and to produce electricity by converting hydrogen and oxygen back into water as needed. Such systems are commonly referred to as regenerative fuel cell systems.
Electrochemical cell systems typically include one or more individual cells arranged in a stack, with the working fluids directed through the cells via input and output conduits formed within the stack structure. The cells within the stack are sequentially arranged, each including a cathode, a proton exchange membrane, and an anode (hereinafter xe2x80x9cmembrane electrode assemblyxe2x80x9d, or xe2x80x9cMEAxe2x80x9d). Each cell typically further comprises a first flow field in fluid communication with the cathode and a second flow field in fluid communication with the anode. The MEA may be supported on either or both sides by screen packs or bipolar plates disposed within the flow fields, and which may be configured to facilitate membrane hydration and/or fluid movement to and from the MEA. In the alternative, or in addition to screen packs or bipolar plates, pressure pads or other compression means are often employed to provide even compressive force from within the electrochemical cell.
While existing electrodes for fuel cells or electrolysis cells are suitable for their intended purposes, there still remains a need for improvements, particularly regarding electrode catalyst compositions that oxidize under high anodic potentials. Electrodes 114, 116 (in either fuel cells or electrolysis cells) conventionally comprise a catalyst material such as a precious metal. However, in certain electrochemical cells, electrodes are provided with less catalyst loading to decrease cost. Such electrodes include a catalyst, a proton conductor such as a perfluoroionomer, and a diluent or support such as particulate carbon. The precious metal provides the active electrocatalytic site, the proton conductor provides a proton xe2x80x9cbridgexe2x80x9d communicating protons between the precious metal and the proton exchange membrane, and the carbon provides a high surface area for attachment of the catalyst materials and the proton conductor. U.S. Pat. No. 5,234,777 to Wilson describes a solid polymer electrolyte membrane assembly wherein a film of a proton conducting material or binder has a supported platinum catalyst dispersed therein, and where the film is bonded to the membrane. U.S. Pat. No. 5,227,042 to Zawodzinski, et al. discloses use of a carbon-supported catalyst wherein the catalyst may be composed of precious metals such as platinum. However, the carbon conventionally employed as the diluent is readily oxidizable in the electrochemical cell environment because of the high anodic potentials applied, which are generally greater than about 1.5 volts.
Accordingly, there exists a need for a non-oxidizable material to support catalyst and proton conductive materials and form an electrode for use in an electrochemical cell environment.
The above-described drawbacks and disadvantages of the related art are alleviated by an electrode for use in an electrochemical cell, its methods of manufacture, a membrane electrode assembly formed thereby, and an electrochemical system employing the electrode. Based on the total weight of the electrode, the electrode comprises about 5 to about 95 wt. % of a support material that is non-oxidizable at anodic potentials of less than about 4 volts; about 5 to about 95 wt. % of a catalyst disposed on the support; and up to about 50 wt. % of a proton conductive material disposed with the catalyst.
In one embodiment, a method of manufacturing an electrode for an electrochemical cell comprises sintering, melt extruding, or casting a composition comprising a catalyst material and a support material that is non-oxidizable at anodic potentials of less than about 4 volts. A membrane electrode assembly may be formed by contacting the sintered, mixed, and extruded or cast composition with a proton conductive material.
In another embodiment, a method of manufacturing a membrane electrode assembly for an electrochemical cell comprises sintering, melt extruding, or casting a composition comprising a non-oxidizable support material, a catalyst material, and a proton conductive material.
The electrochemical cell comprises a first electrode that is non-oxidizable at anodic potentials of less than about 4 volts, wherein the electrode comprises about 5 to about 95 wt. % of a support, about 5 to about 95 wt. % of a catalyst disposed on the support, and up to about 50 wt. % of a proton conductive material disposed with the catalyst; a second electrode; and a membrane disposed between and in intimate contact with the first electrode and second electrode. An electrochemical cell system further comprises a first flow field in fluid communication with the first electrode opposite the membrane; a second flow field in fluid communication with the second electrode opposite the membrane; a water source in fluid communication with the first flow field; and hydrogen removal means in fluid communication with the second flow field.